I have two identical computers . . . same mobo, same RAM, same HDDs, same video, etc., etc. But, if I take a adrive from one to the other, it will "find new hardware" all over the place . . . controllers, etc. That tells me there is more to ID than same general hardware . . . there have to be differences that are sensed regarding lot numbers, serials, batches, etc.
But, can the same copy of XP be used on more than one . .. laptop plus the parent desktop is the rule. However . . . that depends on what edition you have. Academic and corporate have different rules and are individually more open.